Acetaldehyde, also known as Ethanal, is an organic chemical compound. Acetaldehyde Formula is \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CHO}}\), sometimes abbreviated by chemists as \({\rm{MeCHO}}\) (\({\rm{Me = }}\) methyl). It is one of the most important aldehydes, occurring widely in nature and being produced on a large scale. Two carbon atoms are commonly found in this chemical molecule, with five single bonds and one carbon-oxygen double bond. 

Plants create acetaldehyde, which can be found in coffee, bread, and ripe fruit. It’s also produced when the liver enzyme alcohol dehydrogenase partially oxidises ethanol, and it’s one of the main causes of hangovers.

Acetaldehyde Formula

The IUPAC name of Acetaldehyde is Ethanal with a condensed chemical formula \({{\rm{C}}_2}{{\rm{H}}_4}{\rm{O}}.\) The functional group present in it is an aldehyde; −\({\rm{CHO}}\) and it is made up of two carbon atoms, four hydrogen atoms, and one oxygen atom.

Molar Mass of Acetaldehyde

The molar mass of Acetaldehyde, \({\rm{C}}{{\rm{H}}_{\rm{3}}}{\rm{CHO}}\) is:

\( = {\rm{2x}}\left( {{\rm{The}}\,{\rm{atomic}}\,{\rm{massof}}\,{\rm{carbon}}} \right) + 4{\rm{x}}({\rm{The}}\,{\rm{atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{hydrogen}}) + {\rm{Atomic}}\,{\rm{mass}}\,{\rm{of}}\,{\rm{Oxygen}}\)

\({\rm{ = 2x(12}}{\rm{.01) + 4x}}\left( {{\rm{(1}}{\rm{.007) + 15}}{\rm{.999 = 44}}{\rm{.05\;g\;mo}}{{\rm{l}}^{{\rm{ – 1}}}}} \right.\)

Hence, one mole of Acetaldehyde weighs \({{\rm{44}}{\rm{.05}}}\) grams.

Hybridisation of Acetaldehyde

Acetaldehyde has one methyl group attached to a \({\rm{( – CHO)}}\) group at the \({\rm{C – 1}}\) position. The carbon of the terminal methyl group is \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised, whereas the carbon atom of the aldehyde group at the \({\rm{C – 1}}\) position is \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised.

Steric number of the aldehydic carbon atom \( = \) Number of atoms attached \( + \) Lone pairs

\({\rm{ = 3 + 0 = 3}}\left( {{\rm{s}}{{\rm{p}}^{\rm{2}}}} \right)\)

Steric number of the methyl carbon atom \( = \) Number of atoms attached \( + \) Lone pairs

\({\rm{ = 4 + 0 = 4}}\left( {{\rm{s}}{{\rm{p}}^{\rm{3}}}} \right)\)

The carbonyl carbon atom of Ethanal is \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised \(\left( {{\rm{1s + 2P = 3s}}{{\rm{p}}^{\rm{2}}}} \right)\). This means the atomic orbitals of the carbon atom undergo intermixing to form \({\rm{3s}}{{\rm{p}}^{\rm{2}}}\) hybridised orbitals.

The \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised carbon forms \(3\) sigma bonds. The formation of \(3\) sigma bonds gives Ethanal a basic trigonal shape with bond angles of \(120\) degrees. Only two out of three \({\rm{p}}\) orbitals of carbon participate in hybridisation; hence, one \({\rm{p}}\) orbital is unhybridized. This unhybridized \({\rm{p}}\) orbital forms a \({\rm{pi}}\) bond with the unhybridized \({\rm{p}}\) orbital of the oxygen atom. This \({\rm{p}}\) orbital is directed above and below the plane of the paper.

Molecular Geometry of Acetaldehyde

In Acetaldehyde, the \({\rm{C}}\) atom from the aldehyde has a \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridisation, but the methyl group has \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridisation, thus the acetaldehyde molecule has a planar-trigonal together with tetrahedral geometry.

Bond Angle of Acetaldehyde

The \({\rm{ – CHO}}\) group at the \({\rm{C – 1}}\) position is \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised, and the terminal methyl groups are \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised. The \({\rm{O – C – H}}\) bond angle at \({\rm{C – 1}}\) should be approx \({\rm{12}}{{\rm{0}}^{\rm{o}}}\), and the terminal methyl group is in tetrahedral shape, the bond angle within the methyl groups is \({\rm{109}}{\rm{.}}{{\rm{5}}^{\rm{o}}}\).

Dipole Moment of Acetaldehyde

In Acetaldehyde, the main component of the dipole moment is the \({\rm{C = O}}\) bond of the \({\rm{ – CHO}}\) group. This is because:

1. The terminal methyl group being tetrahedral, has zero dipole moment.
2. Similar atoms, i.e. \({\rm{C – C}}\) bond does not contain a dipole.
3. An electronegativity difference exists between carbon and oxygen; hence the \({\rm{C = O}}\) bond is polar in nature.

As there is only one polar bond, no cancelling can happen, and the dipole moment of the entire acetaldehyde molecule is due to the polar nature of the \({\rm{C = O}}\) bond. The dipole moment of Acetaldehyde is found to be \(2.5\) Debye.

Lewis Structures of Acetaldehyde

Ethanal, consisting of an aldehyde functional group, comprises two Carbon atoms with an Oxygen atom double-bonded to the \({\rm{C – 1}}\) Carbon atom. In the Lewis structure for Acetaldehyde, there are a total of \(18\) valence electrons. These electrons are distributed as shown below:

Skeletal Formula of Acetaldehyde

Acetaldehyde or Ethanal is a two-carbon compound with an aldehyde functional group at the \({\rm{C – 1}}\) position. The methyl carbon has single bonds to three \({\rm{H}}\) atoms and one \({\rm{C}}\) atom; the aldehydic carbon has single bonds to \({\rm{C}}\) and \({\rm{H}}\) and a double bond to Oxygen atom. It has six sigma bonds and one \({\rm{pi}}\) bond. The skeletal structure of Acetaldehyde is shown below:

Three Dimensional Representation of Acetaldehyde

The three-dimensional structure of an organic compound is represented by the Wedge-dash method. The \({\rm{3 – D}}\) structure of Ethanal has the following aspects-

1. A bond that protrudes out of the plane of paper towards the viewer is represented by a solid wedge.
2. A bond that projects away from the viewer or into the plane of the paper is represented by a dashed wedge, and
3. A line is used to represent the bond that lies in the plane of the paper.

Considering the above factors, Acetaldehyde can be represented as follows-

Preparation of Acetaldehyde

The preparation of Acetaldehyde is explained below:

Laboratory Preparation of Acetaldehyde

Acetaldehyde is prepared in the Laboratory by the oxidation of ethanol with acidified potassium dichromate solution.

Manufacture of Acetaldehyde

1. By the oxidation of ethanol in the presence of a silver catalyst.

2. By the hydration of acetylene in which mercury \(\left( {{\rm{II}}} \right)\) salts serve as a catalyst:

 The reaction is conducted at a temperature of \({\rm{90 – 95}}{\,^{\rm{o}}}{\rm{C}}\), and the formed Acetaldehyde is separated from water and mercury and cooled to \({\rm{25 – 30}}{\,^{\rm{o}}}{\rm{C}}.\)

3. By dehydrogenation of ethanol in the presence of copper as a catalyst.

In this process, ethanol vapour is passed at a temperature of \(260 – 290{\,^{\rm{o}}}{\rm{C}}.\)

4. By Wacker’s process, oxidation of ethylene in the presence of Palladium\(\left( {{\rm{II}}} \right)\) chloride and Copper\(\left( {{\rm{II}}} \right)\) chloride.

Physical Properties of Acetaldehyde

	Properties
Appearance	Colourless gas or liquid
Odour	Ethereal
Density	\({\rm{0}}.{\rm{784}}\;{\rm{g}} \times {\rm{c}}{{\rm{m}}^{{\rm{ – 3}}}}\left( {{\rm{20}}{\,^{\rm{o}}}{\rm{C}}} \right)\)
Melting Point	\({\rm{ – 123}}.{\rm{37}}{\,^{\rm{o}}}{\rm{C}}\)
Boiling Point	\(20.2{\,^{\rm{o}}}{\rm{C}}\)
Solubility	miscible with water, ethanol, ether, benzene, toluene, xylene, turpentine, acetone, slightly soluble in chloroform
	Structure
Molecular Shape	trigonal planar \(\left( {{\rm{s}}{{\rm{p}}^{\rm{2}}}} \right)\) at \({{\rm{C}}_{\rm{1}}}\) tetrahedral \(\left( {{\rm{s}}{{\rm{p}}^{\rm{3}}}} \right)\) at \({{\rm{C}}_{\rm{2}}}\)
Dipole Moment	\(2.5\,{\rm{D}}\)

Chemical Properties of Acetaldehyde

• 1. Acetaldehyde is polarised due to the \( – {\rm{C}} = {\rm{O}}\) bond present at \({\rm{C}} – 1\).
• 2. The presence of positively charged centre nucleophilic molecules attack the carbonyl carbon which is deficient in electrons. At the same time, electrophiles attack the negatively charged oxygen atom.

• 3. Acetaldehyde tautomerizes to give vinyl alcohol (enol).
• 4. Acetaldehyde undergoes condensation with amines to yield imines. These imines can be used to direct subsequent reactions like an aldol condensation.
• 5. Acetaldehyde is a precursor to vinyl phosphonic acid, used to make adhesives.
• 6. With Grignard reagent, Acetaldehyde produces hydroxy-ethyl derivatives and is an important building block in the synthesis of heterocycles such as imines and pyridines.

Uses of Acetaldehyde

1. Acetaldehyde is next to formaldehyde in the homologous aldehyde series and is an important precursor in organic synthesis.
2. It is used in the production of dyes, perfumes, acetic acid and as a flavouring agent.
3. It is also used as a solvent in industrial processes such as paper manufacture, tanning, and rubber.
4. A useful resin is produced when Acetaldehyde combines with urea.

Summary

Acetaldehyde finds its use in a wide range of applications, from flavouring agents to superabsorbents in baby nappies. It acts as the base material for the production of acetic acid. To understand its importance, it is essential to learn about its structure and related properties. Through this article, we learnt the basic structure, properties and their preparation. We also learned its variety of uses.

PRACTICE QUESTIONS RELATED TO ACETALDEHYDE

FAQs

Q.1. What is Acetaldehyde?
Ans: Acetaldehyde is Ethanal with the chemical formula \({\rm{C}}{{\rm{H}}_3}{\rm{CHO}}\). It ranks second in the homologous aldehyde series. It is a two-carbon compound with the aldehyde functional group at the \({\rm{C – 1}}\) position. The methyl carbon has single bonds to three \({\rm{H}}\) atoms and one \({\rm{C}}\) atom; the aldehydic carbon has single bonds to \({\rm{C}}\) and \({\rm{H}}\) and a double bond to \({\rm{O}}\). It has six sigma bonds and one pi bond. The methyl carbon atom is \({\rm{s}}{{\rm{p}}^{\rm{3}}}\) hybridised, whereas the aldehydic carbon atom is \({\rm{s}}{{\rm{p}}^{\rm{2}}}\) hybridised.

Q.2. What is the common name for Acetaldehyde?
Ans: IUPAC name of Acetaldehyde is Ethanal.

Q.3. What drinks are high in Acetaldehyde?
Ans: Beverages such as tea and soft drinks have high acetaldehyde concentrations. For example- Soft drinks \(\left( {{\rm{0}}.{\rm{2 – 0}}.{\rm{6}}\,{\rm{ppm}}} \right)\), beer \((0.6 – 24\,{\rm{ppm}})\), wine \((0.7 – 290\,{\rm{ppm}})\) and spirits \((0.5 – 104\,{\rm{ppm}})\).

Q.4. Does Acetaldehyde make you drunk?
Ans: When we drink acetaldehyde, our liver turns Acetaldehyde into an acid. Some of the Acetaldehyde enters our blood, damaging our membranes. It also leads to a hangover and can result in a faster heartbeat, a headache or an upset stomach.

Q.5. Does Acetaldehyde give an iodoform test?
Ans: Yes, Acetaldehyde gives a positive iodoform test. It contains the \({\rm{C}}{{\rm{H}}_3}{\rm{C}} = 0\) group and is the only aldehyde that gives the iodoform test.

Q.6. What is the difference between ethanol and Ethanal?
Ans: Ethanol is a two-carbon compound with a hydroxyl \(( – {\rm{OH}})\) functional group, whereas Ethanal is a two-carbon compound with an aldehyde \(( – {\rm{CHO}})\) functional group. Ethanol has the chemical formula \({\rm{C}}{{\rm{H}}_3}{\rm{C}}{{\rm{H}}_2}{\rm{OH}}\), whereas the chemical formula for Ethanal is \({\rm{C}}{{\rm{H}}_3}{\rm{C}} = {\rm{O}}\).

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